Add TTT as an optional terminal editor

[BrettKinny]

Adds TTT (Terminal Text Tool) as an optional editor in interactive setup, saved Selections, and Dev Container provisioning. Users can select ttt as their default editor and update it through sqrbx-update.

Installation uses the shared fail-closed installer with GitHub's exact release-asset SHA-256 digest for Linux amd64 and arm64 binaries. Includes documentation, optional-tool E2E coverage, and regressions for failed installations and preserving the saved TTT default during reconciliation.

Validation:

• Verified a real installation of TTT v1.7.1 through the shared installer.
• 20 executable shell test suites passed, including provisioning, tool library, updater, and version-refresh coverage.
• test-lifecycle-install-state.sh stopped with an OS PermissionError: Operation not permitted.
• Docker build and smoke validation could not run because access to the Docker socket was denied.
• Shell syntax and git diff --check passed.